where the mail store when arrive fresh to the server

I'm notice that the file of the user on /var/spool/mail only modified when user read their mail trought a web portal like squirrel, but, until the user don't use squirrel mail the newest mail does'nt appears on /var/spool/mail/user file, so.... anyone knew where the new incoming mail storage until user see their mail trought squirrel?? i'm trying to make a shell script that count the mail storage on server and more